1. Field of the Invention
The invention relates generally to a control apparatus and method for a shift-position changing mechanism, which changes shift-positions of an automatic transmission using an actuator. More specifically, the invention relates to a control apparatus and method for a shift-position changing mechanism, which determines whether a failure has occurred in an automatic transmission.
2. Description of the Related Art
There is a shift-position changing mechanism which changes shift positions (hereinafter, sometimes referred to as “shift ranges”) of an automatic transmission in response to an operation of a shift lever performed by a driver, and which is provided with an electric motor (for example, a direct-current motor) as a power source for changing the shift positions.
With such shift-position changing mechanism, the shift lever and the shift-position changing mechanism need not be mechanically connected to each other, unlike a common changing mechanism that directly changes shift positions of an automatic transmission using an operating force applied to a shift lever by a driver. This eliminates the limitation on the layout of components in a vehicle, thereby providing greater flexibility to the vehicle design. In addition, the shift-position changing mechanism thus structured is more easily fitted to the vehicle.
With this type of shift-position changing mechanism, when a failure occurs in the automatic transmission, a failsafe process needs to be executed promptly.
For example, Japanese Patent Application Publication No. 05-223156 (JP-A-05-223156) describes a “Shift-by-Wire” shift control unit for an automatic transmission, which reliably brings a vehicle into the limp home running mode that is appropriate for the road condition when a failure occurs. The “Shift-by-Wire” shift control unit includes a hydraulic sensor that detects the hydraulic pressure in a range-setting oil passage, range determination means for determining the shift range based on the hydraulic pressure detected by the hydraulic sensor, on-off state setting means for setting a combination of on-off states of solenoid valves, which is used to set the selected shift range, output means for transmitting signals to the solenoid valves in order to bring the solenoid valves into the on-off states indicated by the combination which is set by the on-off state setting means, and failure determination means. When the shift range determined by the range determination means differs from the selected shift range, the failure determination means instructs the on-off state setting means to change the combination of the on-off states of the solenoid valves in order to set the selected shift range.
The above-described “Shift-by-Wire” shift control unit determines that, a failure has occurred, when a hydraulic pressure is not generated in the range-setting oil passage that is used to set the selected shift range. When a failure has occurred, the shift control unit changes the combination of the on-off states of the solenoid valves, which is used to set the selected shift range, to another combination, and executes the on-off control over the solenoid valves. Accordingly, even if a failure, for example, valve-sticking occurs, it is possible to select and set the shift range. Therefore, it is possible to bring the vehicle into the limp home running mode under any road conditions.
However, because the above-described “Shift-by-Wire” shift control unit determines the shift position based on the hydraulic pressure detected by the hydraulic sensor, the timing at which the shift position is determined is sometimes delayed. This is because, when the temperature of the hydraulic fluid is low, the viscosity of the hydraulic fluid increases and therefore the timing at which the hydraulic pressure starts increasing or decreasing is delayed. If the timing at which the shift position is determined is delayed, the timing at which the failsafe operation is started may also be delayed.